Energy comes in many forms. Some forms are more obvious than others, such as the thermal energy of hot coffee, or the electrical energy that runs our homes and offices.
Less obvious as a form of energy is vibration, which is a form of mechanical energy. Vibration comes in the form of sound, the shaking of machinery, or the invisible radio frequencies around us.
Sources of energy, among others, include the following:                Vibration of equipment or the motion of a human;        Oscillation of a magnetic field; and        Cyclical motions present in nature.        
The energy from these sources can be made more useful, or measurable, by converting the mechanical vibrational energy into electrical energy. Such a conversion between types of energy is performed by an energy harvester. The harvester can also be considered a sensor, as it can sense mechanical energy and relay the energy as a proportional electrical output. The harvester can also be a type of transducer as it converts one form of energy into another.
The harvester can also be operated in reverse—electrical energy applied to the same device creates mechanical motion, and thereby making the same device an actuator too. Henceforth, for simplicity and clarity, energy harvester will be synonymous with “sensor” or “transducer.”
Existing energy harvesters are unable to harvest low-frequency vibrations without the use of a tip mass, or without sacrificing compactness.
What is needed is a micro-scale energy harvester that can harvest low-frequency vibrations, while maintaining a small form factor.